The Red Sox have now lost six of their past seven games.
Boston starter Jon Lester lasted seven innings and allowed three runs on six hits with two walks and nine strikeouts. He tossed a total of 107 pitches (67 strikes).
It was a brutal homestand for the Red Sox, and now they begin a road trip through Miami to face the Marlins and Chicago to face the Cubs.
Red Sox manager Bobby Valentine was ejected in the bottom of the ninth inning for arguing balls and strikes when Dustin Pedroia was at the plate with two outs.
ABOVE AND BEYOND: Red Sox DH David Ortiz entered Sunday's game with a .300 average, and if he hadn't doubled in his first at-bat, his average would have dipped below .300 for the first time this season. He then followed up that extra-base hit with his 14th homer of the season, a solo shot to lead off the bottom of the fourth inning. Ortiz finished the day 3-for-4.
BELOW GRADE: Red Sox first baseman Adrian Gonzalez went 0-for-4 on Sunday and finished the homestand 6-for-25. He also went 0-for-4 on Saturday.
HEADS UP: Valentine decided to give third baseman Kevin Youkilis and shortstop Mike Aviles the day off on Sunday. Rookie Will Middlebrooks got the start at third and Nick Punto started at shortstop. Punto showcased his solid defensive skill set on a few plays, including a heads-up play in the top of the fourth inning. With one out and a runner on second, the Nationals' Michael Morse hit a ground ball to short. Washington baserunner Ian Desmond started toward third before retreating, and Punto made a quick throw to second baseman Pedroia, who put the tag on Desmond.
WEB GEM: Red Sox right fielder Ryan Sweeney turned in the defensive play of the game with a spectacular catch in the top of the sixth inning. The Nationals' Ryan Zimmerman lifted a high fly ball into the corner just past the Pesky Pole. Sweeney, in an all-out sprint, made it to the corner and made a sliding catch for the first out of the inning. Sweeney, however, failed to hit the cut-off man on a throw in the top of the ninth inning, allowing the game-winning run to score.
SCRATCHED: Sunday's original lineup for the Sox had outfielder Daniel Nava leading off, but he was a last-minute scratch due to a sore left hand. Darnell McDonald was inserted into left field and batted ninth, while center fielder Scott Podsednik led off for the Sox.
SELLOUT STREAK: The Red Sox now officially own the longest regular-season sellout streak in major professional sports history. The 37,467 in attendance Sunday at Fenway Park extended the streak to 745 consecutive games, surpassing the previous record of 744, which was held by the Portland Trailblazers of the NBA from 1977 to 1995. Boston's streak began on May 15, 2003.
UP NEXT: The interleague schedule continues as the Red Sox travel to Miami to begin a three-game series against the Marlins on Monday night at the new Marlins Park. Josh Beckett (4-6, 4.04 ERA) will start for the Red Sox, while Josh Johnson (3-4, 4.56) gets the nod for the Marlins.